This invention relates, in general, to semiconductor devices, and more particularly to a metallization process for semiconductor devices wherein hillocking is greatly reduced.
Conventional metallization processes for semiconductor devices deposit metals at low temperatures and slow deposition rates. For example, conventional sputtering techniques using D.C. magnetron deposit aluminum alloys at approximately 250 degrees centigrade and 20 angstroms per second. Because these conventional methods use such low deposition temperatures, subsequent processing steps such as plasma oxide (approximately 300 degrees centigrade) and polyimide anneals (as high as 450 degrees centigrade) are performed at temperatures higher than those used in the metal deposition. The higher temperatures of the subsequent processing steps create stress throughout the metallization thereby causing severe hillocking. One skilled in the art will recognize that hillocks are protrusions of the metal caused by stress. Hillocking is detrimental in that it creates severe shorting problems between metal layers.
In semiconductor devices, metal is commonly deposited on interlayer dielectrics. Although interlayer dielectrics may be comprised of various materials, polyimide has been found to have many unique properties and characteristics such as planarization of the underlying structure which render it extremely valuable as an interlayer dielectric. However, it is extremely difficult to deposit metal on polyimide because excessive heat causes it to wrinkle. Poor electromigration characteristics are another fault of conventional metal deposition techniques. Because the electric current flowing through the metal of a semiconductor device may displace patches of metal, continuity of metal lines may be greatly affected and reduce the useful lifetime of the semiconductor device. Accordingly, a metallization process for semiconductor devices having reduced hillocking, enhanced electromigration characteristics and compatibility with interlayer dielectrics comprised of polyimide would be extremely useful.